Before installing the deck sections and greenroof panels, workers laid a landscape fabric slip sheet over the waterproofing layer. The slip sheet allows water to drain through and helps protect the EPDM membrane.

These green-roof panels contain a drought-resistant mixture of sedum and thyme. As the root systems spread, the seams between the panels will disappear.

We ordered panels planted with a mix of sedum and thyme and picked them up from a distributor on Long Island. Sedum is a prolific and hardy water-storing succulent; because it’s drought-tolerant, it’s often used instead of grass on green roofs. Creeping thyme is a low-growing herb that adds a little fragrance to the garden. (We hope eventually to work primarily with plant-in-place panels that we’ve seeded in our own small greenhouse, which will allow us to vary our plantings and control the delivery schedule.)

The dimpled HDPE membrane used for the base layer of these panels has considerable compressive strength (the same material is used as a drainage layer in road construction), so features like planters, paving, and decking can be placed directly on the panels after they’ve been installed. This is the typical approach on large roofs where planters and pavers occupy a small percentage of the surface area. On a small project like this one, where those features cover a large portion of the deck’s footprint, we saved material by first installing the ipe deck, stepping pads, and planters, then cutting the panels to fit using a cut-off saw with an abrasive blade. Prevegetated panels are about 95 percent planted, with the eggcrate-like reservoir layer exposed along two of the edges. When the panels are installed, their edges overlap, which locks them together. It takes only a month or two before roots completely fill in the small gaps between panels. As the plantings mature, the seams disappear — though one of the advantages of this system is that individual panels (or the entire roof) can be removed if repairs to the waterproofing layer are ever required.

Green roofs are normally installed with automatic irrigation. But in our reasonably wet Upstate New York climate — and with the water-retention capabilities of the panels — we expect that the homeowner will be able to handle any supplemental watering with a garden hose. This spring we plan to connect rain-barrel collectors to the gutter system to help monitor how much water actually flows off the roof (some systems pump collected water back up to the roof when irrigation is needed).

Cost

We completed the project in October, relatively late in the area’s growing season, which gave the root system little time to establish itself before first frost. Still, we don’t anticipate having to do any reseeding this year. Total cost was $7,214, or about $35 per square foot, with materials accounting for about $5,000 of that total. The square-foot cost was high because of the scale of the project (200 square feet); costs for a more typical green roof project using this system are usually in the $10- to $20-per-square- foot range.